The rejection of heat in space is a critical aspect of virtually all proposed space-borne installations, from solar power satellites to low temperature materials processing laboratories. On purely thermodynamic grounds it is desirable to reject heat at as low a temperature as possible, for instance, to maximize efficiency in a power cycle or to minimize the work required to drive a heat pump, and this implies a large radiator area and mass. This obviously conflicts with the basic requirement that any space-based installation has minimal mass. The balance between device performance and radiator mass thus forms a central design problem for many space systems.
In most designs the radiator is composed of an array of tubes or tube-fin structures through which flows a coolant. The tubes must be sufficiently massive to minimize micrometeorical penetration; in addition, transport of the coolant over large distances is often required. Radiator mass in such designs often comprises a large fraction of the total system mass.
J. M. Hedgepeth proposed, in "Ultralightweight Structures for Space Power," in Radiation Energy Conversion in Space, Vol. 61 of Progress in Astronautics and Aeronautics, K. W. Billman, ed. (AIAA, New York, 1978), p. 126, the use of a dust radiator to reduce the radiator weight of solar power satellites. A cylindrical column of dust particles is heated radiatively by the working fluid of a thermal engine and is then sent on a 100 m to 10,000 m trajectory to radiate energy into space and finally is collected, reheated, and redirected. While this idea holds promise for sizeable weight reduction, it has significant practical problems, such as the inefficiency of heating the dust by radiation, the difficulty of manipulating a stream of dust, and degradation of the dust itself over time.
The concept of using a stream of liquid droplets as a lightweight radiator for space retains the low-mass advantages of a dust radiator and has the additional advantages of allowing heat transfer by conductor (heat exchanger) and ease of manipulation. To indicate the degree of improvement possible using droplets instead of tube and fin structures to radiate heat, it is noted that radiator performance is characterized primarily by the specific mass, i.e., mass per radiator area. The best tube and fin designs incorporating heat pipes have specific masses of 5-10 kg/m.sup.2. The specific mass of a droplet is simply 1/3 pa, where a is the droplet radius. Even with a medium as heavy as liquid tin (p=6.8 g/cm.sup.3), which turns out to be an excellent medium for rejection in space thermal engines, the specific mass is 0.1 kg/m.sup.2 for 0.1 mm diameter droplets, a 50-fold improvement over tubefin radiators.
A recent advance in the construction of liquid droplet radiators is directed to the collector portion disclosed in U.S. Pat. No. 4,702,309 in the name of the present inventor and assigned to the present assignee. Although that disclosed collector portion of the radiator may be considered to operate satisfactorily, problems have been discovered in connection with the droplet generator which limits the usefulness thereof. In particular, a liquid is forced through tiny orifices to generate droplets which are then directed to the radiator collector. During continued use of existing generators, it has been found that liquid drips from the outlet of the orifices each time the liquid flow is turned off and this creates a contaminating liquid source for surrounding structures.
Accordingly, there is a need to sharply shut off the liquid flow from the generator without creating contaminating liquid drips.